What Is a Genetic Disorder?

A genetic disorder is a health condition caused wholly or partly by changes in a person's genetic material.

These changes may involve:

• a single gene
• several genes
• part of a chromosome
• an entire chromosome

Some genetic disorders are inherited from biological parents.

Others result from new genetic changes that occur during the formation of gametes or early:

development.

Therefore:

genetic does not always mean inherited.

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Genes, DNA, and Proteins

To understand genetic disorders, we first need to remember the relationship between:

DNA → genes → proteins → characteristics.

A gene is a section of DNA containing information used to produce a functional product, often a:

protein.

Proteins perform many important functions in the body.

They can act as:

• enzymes
• structural components
• receptors
• transport proteins
• signaling molecules

A change in DNA can sometimes alter how one of these proteins works.

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What Is a Mutation?

A mutation is a change in the DNA sequence.

Mutations can occur:

spontaneously

or as a result of exposure to certain environmental:

mutagens.

Mutations create new genetic variation.

Their effects can be:

harmful

neutral

or occasionally:

beneficial.

Most importantly:

not every mutation causes a genetic disorder.

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How Can a Mutation Affect a Gene?

Consider a gene as a sequence of DNA instructions.

If the DNA sequence changes, the instructions may also:

change.

This can affect:

RNA production

and potentially:

protein production or function.

A simplified pathway is:

DNA mutation → altered gene product → altered cell function → possible phenotype

However, the outcome depends on the type and location of the mutation.

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Mutations Can Have Different Effects

A mutation might:

• have no detectable effect
• change one amino acid
• produce a shortened protein
• prevent a protein from being produced
• alter how much protein is produced
• change when or where a gene is active

Therefore, mutations do not all affect organisms in the same:

way.

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Substitution Mutations

A substitution occurs when one DNA base is replaced by:

another.

For example:

Original:

A T G C C A

Mutated:

A T G T C A

One base has been:

substituted.

Depending on its location, this change may have little effect or may alter the resulting protein.

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Insertion Mutations

An insertion occurs when one or more DNA bases are added to a:

sequence.

For example:

Original:

A T G C C A

After insertion:

A T G A C C A

Insertions within a protein-coding sequence can sometimes alter the reading frame of the gene.

This can have a major effect on the resulting:

protein.

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Deletion Mutations

A deletion occurs when one or more DNA bases are:

removed.

For example:

Original:

A T G C C A

After deletion:

A T C C A

Like insertions, some deletions can alter how the genetic sequence is read.

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Germline and Somatic Mutations

Mutations can occur in different types of:

cells.

A mutation occurring in a cell that gives rise to an egg or sperm can potentially be passed to:

offspring.

These are associated with the:

germline.

A mutation occurring in an ordinary body cell is called a:

somatic mutation.

Somatic mutations are generally not inherited by a person's children.

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Inherited Genetic Disorders

An inherited genetic disorder results from a genetic variant passed from biological parent to:

offspring.

Different disorders can follow different inheritance patterns.

These include:

• autosomal recessive
• autosomal dominant
• X-linked
• mitochondrial inheritance

Some genetic conditions involve more complicated inheritance.

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Autosomal Recessive Disorders

In an autosomal recessive disorder, an individual typically needs two disease-associated recessive alleles for the condition to:

develop.

We can represent a simplified example as:

A = typical allele

a = disorder-associated recessive allele.

Then:

AA → unaffected

Aa → carrier

aa → affected.

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What Is a Carrier?

A carrier has one copy of a recessive disorder-associated allele but usually does not show the disorder associated with having two:

copies.

For example:

Aa = carrier.

A carrier can still pass the recessive allele to their:

offspring.

This is why a recessive disorder can appear in a family even when both parents are unaffected.

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Carrier × Carrier

Suppose:

Aa × Aa.

The predicted probabilities are:

25% AA — unaffected, non-carrier

50% Aa — unaffected carrier

25% aa — affected.

These are probabilities for each pregnancy, not guaranteed family outcomes.

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Cystic Fibrosis

Cystic fibrosis (CF) is an example of an autosomal recessive genetic disorder.

It is caused by disease-associated variants in the:

CFTR gene.

CFTR normally produces a protein involved in the movement of chloride ions across cell:

membranes.

Certain variants reduce or disrupt CFTR protein function.

This can lead to unusually thick mucus affecting organs including the:

lungs and digestive system.

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Sickle Cell Disease

Sickle cell disease is another inherited genetic condition.

It involves variants in the:

HBB gene,

which contains instructions for part of:

hemoglobin.

Hemoglobin is the protein in red blood cells responsible for transporting:

oxygen.

Certain HBB variants produce an altered form of hemoglobin.

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How Sickle Cell Disease Affects Red Blood Cells

Under certain conditions, the altered hemoglobin can cause red blood cells to become:

rigid and sickle-shaped.

These cells can:

• break down more quickly
• interfere with blood flow
• reduce oxygen delivery
• contribute to episodes of severe pain and other complications

Sickle cell disease demonstrates how a change in one gene can influence:

protein structure → cell structure → body function.

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Autosomal Dominant Disorders

In an autosomal dominant disorder, one copy of a disease-associated dominant allele can be sufficient for the condition to:

occur.

Suppose:

H = disorder-associated dominant allele

h = unaffected allele.

Then, in a simplified model:

HH → affected

Hh → affected

hh → unaffected.

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Huntington Disease

Huntington disease is an example of an autosomal dominant genetic:

disorder.

It is caused by a particular type of mutation in the:

HTT gene.

A person with a disease-associated HTT allele can develop progressive neurological:

symptoms.

Because the condition is dominant, an affected heterozygous parent can pass the allele to offspring.

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Dominant Inheritance Example

Suppose an affected heterozygous parent has genotype:

Hh.

The other parent is:

hh.

Cross:

Hh × hh.

Predicted probabilities:

50% Hh — affected

50% hh — unaffected.

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X-Linked Genetic Disorders

Some genetic disorders are caused by variants in genes located on the:

X chromosome.

These are called:

X-linked disorders.

Many commonly studied examples are:

X-linked recessive.

Examples include:

• hemophilia A
• hemophilia B
• Duchenne muscular dystrophy

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X-Linked Recessive Inheritance

Suppose:

Xᴺ = typical allele

Xⁿ = disorder-associated recessive allele.

Possible genotypes include:

XᴺXᴺ → unaffected

XᴺXⁿ → usually carrier

XⁿXⁿ → affected

XᴺY → unaffected

XⁿY → affected.

Because XY individuals normally have only one X chromosome, one disease-associated recessive allele can produce the:

phenotype.

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Hemophilia

Hemophilia is a group of disorders in which blood does not clot normally.

Hemophilia A involves variants affecting:

clotting factor VIII.

Hemophilia B involves variants affecting:

clotting factor IX.

Both are commonly inherited as:

X-linked recessive disorders.

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Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is usually caused by disease-associated variants in the:

DMD gene.

This gene provides instructions for producing:

dystrophin.

Dystrophin is important for maintaining muscle cell:

structure.

Without enough functional dystrophin, muscle cells become progressively damaged.

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Chromosomal Disorders

Not all genetic disorders result from mutations in a single:

gene.

Some result from changes in:

chromosome number

or:

chromosome structure.

These are often called:

chromosomal disorders or chromosomal conditions.

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Nondisjunction

During meiosis, chromosomes normally separate so that gametes receive the correct number of:

chromosomes.

Sometimes chromosomes fail to separate correctly.

This is called:

nondisjunction.

Nondisjunction can produce gametes containing:

an extra chromosome

or:

a missing chromosome.

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Down Syndrome

Down syndrome usually results from having three copies of chromosome:

21.

This is called:

trisomy 21.

Instead of two copies of chromosome 21, most cells contain:

three.

Down syndrome therefore provides an example of a genetic condition caused by a change in chromosome number rather than a single altered gene.

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Gene Disorder vs Chromosomal Disorder

Consider the difference:

Cystic fibrosis

primarily involves variants in one gene:

CFTR.

Down syndrome

usually involves an extra copy of an entire chromosome:

chromosome 21.

Both are genetic conditions, but their genetic causes are very:

different.

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Genetic Does Not Always Mean Inherited

This distinction is extremely important.

A genetic condition involves a change in genetic:

material.

An inherited condition involves genetic information passed from biological parent to:

offspring.

A genetic change can arise for the first time in an egg, sperm, or early embryo.

Therefore:

a genetic disorder can occur without being inherited from either parent.

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New Mutations

A genetic change that appears for the first time in an individual is often called a:

de novo mutation.

This means the genetic change is:

new.

It may have occurred during:

• formation of an egg
• formation of a sperm
• fertilization
• early embryonic development

Such mutations can explain why a genetic disorder may appear without a previous family history.

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Mutations and Protein Function

One of the most important relationships in genetics is:

Gene → Protein → Function → Phenotype.

Suppose a mutation changes a gene.

The altered gene may produce:

an altered protein.

The altered protein may function:

differently or not at all.

This can change cellular function and potentially produce:

symptoms.

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But Not Every Mutation Changes a Protein

Some mutations occur in locations where they have little or no detectable effect.

Others change DNA without changing the amino acid sequence of a:

protein.

Still others alter gene regulation rather than protein structure.

Therefore:

mutation does not automatically mean disease.

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Genetic Disorders and Pedigrees

A pedigree is a diagram showing how a characteristic appears across generations of a:

family.

Pedigrees can help scientists investigate whether a disorder may follow:

• dominant inheritance
• recessive inheritance
• X-linked inheritance

Patterns across generations provide clues about possible:

genotypes.

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Recognizing Autosomal Dominant Patterns

An autosomal dominant disorder may:

• appear in multiple successive generations
• affect individuals of different sexes
• be transmitted by an affected parent
• occur when only one disease-associated allele is inherited

However, real pedigrees may not always show a perfect pattern because families are small and inheritance involves:

probability.

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Recognizing Autosomal Recessive Patterns

An autosomal recessive disorder may:

• appear in children of unaffected parents
• skip apparent generations
• affect individuals of different sexes
• involve unaffected carriers

Two unaffected carriers can therefore have an affected:

child.

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Recognizing X-Linked Recessive Patterns

An X-linked recessive disorder may:

• occur more frequently in XY individuals
• pass through unaffected XX carriers
• show no direct father-to-son transmission
• appear in sons whose mothers carry the allele

These patterns provide evidence for:

X-linked inheritance.

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Worked Example 1: Recessive Disorder

Two unaffected parents have an affected child.

The disorder is known to be autosomal recessive.

Use:

A = typical allele

a = disorder-associated allele.

The affected child must be:

aa.

Therefore, each parent must have contributed:

a.

Because both parents are unaffected, their likely genotypes are:

Aa and Aa.

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Worked Example 2: Probability

Two carriers have genotypes:

Aa × Aa.

What is the probability their next child will have the disorder?

The probability of:

aa

is:

25%.

Even if they already have an affected child, the probability for another pregnancy remains:

25% under this simple model.

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Worked Example 3: Dominant Disorder

One parent is:

Hh

and the other is:

hh.

What is the probability of an affected child?

Possible offspring:

Hh

and:

hh.

Therefore:

50% affected

and:

50% unaffected.

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Worked Example 4: X-Linked Disorder

A carrier mother is:

XᴺXⁿ.

The father is:

XᴺY.

The possible offspring are:

XᴺXᴺ

XᴺXⁿ

XᴺY

XⁿY.

Therefore:

25% of all offspring are predicted to be affected.

Among XY offspring specifically:

50% are predicted to be affected.

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Worked Example 5: Chromosome Number

A person's cells contain three copies of chromosome 21.

Is this primarily a single-gene mutation?

No.

This is a chromosome-number difference called:

trisomy 21.

It is associated with:

Down syndrome.

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Genetic Testing

Modern genetics can sometimes identify variants associated with genetic:

conditions.

Genetic testing may involve analyzing:

• specific genes
• groups of genes
• chromosomes
• larger portions of the genome

The appropriate test depends on the condition being:

investigated.

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Genetic Screening

Genetic screening can be used to investigate whether individuals have particular genetic variants or increased risks.

Screening can sometimes identify:

• carriers
• chromosome differences
• variants associated with inherited disorders

However, genetic information can be complex and may not always provide a simple yes-or-no prediction.

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Genetic Counseling

Genetic counseling helps individuals and families understand genetic information.

A genetic counselor may help explain:

• inheritance patterns
• genetic test results
• probabilities
• testing options
• possible implications for family members

The goal is to provide information and support people in making their own informed:

decisions.

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The Impact on Individuals

Genetic disorders can affect individuals in very different:

ways.

Depending on the condition, effects might involve:

• physical health
• mobility
• development
• learning
• sensory function
• daily activities
• medical treatment

The severity and symptoms of a genetic condition can also vary considerably between:

individuals.

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The Impact on Families

A genetic diagnosis can also affect:

families.

Families may need to consider:

• medical appointments
• treatment and care
• financial costs
• emotional support
• genetic testing
• future reproductive decisions
• access to specialist services

Different families may experience these challenges in very different ways.

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Avoiding Genetic Determinism

Genes can strongly influence biological characteristics, but having a particular genetic variant does not always mean a particular outcome is:

certain.

Some genetic variants have:

high penetrance,

while others only increase:

risk.

Environmental factors and other genes can also influence the phenotype.

Therefore, genetic information must be interpreted carefully.

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Genetic Risk Is Not the Same as Genetic Certainty

Suppose a genetic variant increases the probability of developing a condition.

This does not necessarily mean the individual:

will develop it.

Likewise, not having one particular risk variant does not guarantee that a person will never develop the condition.

Probability and certainty are:

different concepts.

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Ethical Considerations

Genetic information can be highly personal.

Important ethical questions include:

• Who should have access to genetic information?
• Should relatives be informed about inherited risks?
• How should genetic privacy be protected?
• How should genetic testing be used?
• How can genetic discrimination be prevented?

Advances in genetics therefore involve both scientific and:

ethical considerations.

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Comparing Types of Genetic Disorders

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Single-Gene vs Chromosomal Conditions

Single-gene disorder

A change involving a particular gene has a major role.

Examples:

Cystic fibrosis

Huntington disease

Chromosomal condition

A larger chromosome change is involved.

Example:

Down syndrome.

Understanding the genetic cause helps explain why different conditions show different inheritance:

patterns.

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Common Mistake: Genetic Means Inherited

Not necessarily.

A condition can result from a new genetic change that was not present in either:

parent.

Remember:

Inherited conditions are genetic, but not all genetic conditions are inherited.

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Common Mistake: Every Mutation Is Harmful

Most DNA changes do not automatically cause:

disease.

Mutations can be:

• neutral
• harmful
• occasionally beneficial

Their effects depend on where they occur and what biological function they affect.

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Common Mistake: Recessive Means Weak

A recessive allele is not necessarily physically weaker or less:

important.

"Recessive" describes how the phenotype is expressed in relation to another:

allele.

It does not describe the severity of a disorder.

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Common Mistake: Dominant Means More Common

A dominant allele is not necessarily more common in a:

population.

Dominant describes:

expression in a heterozygote.

An allele can be dominant and rare.

An allele can also be recessive and relatively common.

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Common Mistake: A 25% Risk Means One in Every Four Children

If a cross predicts a:

25% probability

of a disorder, this does not guarantee that exactly one of every four children will be:

affected.

Each pregnancy represents a new probability event.

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Common Mistake: Everyone With the Same Disorder Is Affected Identically

Symptoms can vary between individuals because of:

• different mutations
• other genes
• environmental factors
• treatment
• age
• biological variation

A diagnosis therefore does not necessarily predict an individual's exact experience.

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Check Your Understanding

1. Define a genetic disorder.

2. What is a mutation?

3. Explain why not every mutation causes disease.

4. Describe the relationship between DNA, genes, proteins, and phenotype.

5. What is a substitution mutation?

6. What is an insertion mutation?

7. What is a deletion mutation?

8. Explain how a mutation could affect protein function.

9. What is the difference between a germline and somatic mutation?

10. Explain the difference between genetic and inherited.

11. What is an autosomal recessive disorder?

12. What is a carrier?

13. Cross two carriers, Aa × Aa. What percentage of offspring are predicted to be affected?

14. What percentage are predicted to be carriers?

15. Explain how two unaffected parents can have a child with an autosomal recessive disorder.

16. Give an example of an autosomal recessive genetic disorder.

17. Explain how cystic fibrosis affects the CFTR protein.

18. Explain how a change in the HBB gene can affect red blood cells.

19. What is an autosomal dominant disorder?

20. Give an example of an autosomal dominant disorder.

21. What is an X-linked genetic disorder?

22. Give two examples of X-linked disorders.

23. Why are X-linked recessive disorders often more common in XY individuals?

24. What is nondisjunction?

25. Explain how nondisjunction can change chromosome number.

26. What chromosomal difference is usually associated with Down syndrome?

27. What is a pedigree?

28. Describe one clue suggesting autosomal dominant inheritance.

29. Describe one clue suggesting autosomal recessive inheritance.

30. Describe one clue suggesting X-linked recessive inheritance.

31. Why can genetic testing be useful?

32. What is genetic counseling?

33. Explain why genetic probabilities do not guarantee outcomes.

34. How might a genetic disorder affect an individual?

35. How might a genetic disorder affect a family?

36. Why is genetic privacy important?

37. Explain why dominant does not mean common.

38. Explain why recessive does not mean mild.

39. Why can people with the same genetic disorder experience different symptoms?

40. Explain the statement: "Inherited disorders are genetic, but genetic disorders are not always inherited."

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Key Terms

• Genetic disorder: Health condition caused wholly or partly by changes in genetic material.
• Mutation: Change in DNA sequence.
• Substitution: Mutation in which one DNA base is replaced by another.
• Insertion: Mutation in which DNA bases are added.
• Deletion: Mutation in which DNA bases are removed.
• Germline mutation: Genetic change occurring in cells that can contribute to eggs or sperm and potentially be inherited.
• Somatic mutation: Mutation occurring in a body cell that is generally not passed to offspring.
• Inherited disorder: Genetic condition resulting from genetic information passed from biological parent to offspring.
• Carrier: Individual possessing a recessive disease-associated allele without usually showing the associated recessive disorder.
• Autosomal recessive: Inheritance pattern in which two disease-associated recessive alleles are typically required.
• Autosomal dominant: Inheritance pattern in which one disease-associated dominant allele may be sufficient.
• X-linked: Involving a gene located on the X chromosome.
• Nondisjunction: Failure of chromosomes to separate normally during cell division.
• Trisomy: Presence of three copies of a particular chromosome.
• Pedigree: Diagram showing inheritance through generations.
• De novo mutation: Genetic change appearing newly in an individual.
• Genetic testing: Analysis of genetic material to investigate genetic variants.
• Genetic counseling: Professional support for understanding genetic information and inheritance.

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Key Takeaways

• Genetic disorders result from changes in genes, chromosomes, or other genetic material.
• A mutation is a change in DNA.
• Mutations can involve substitutions, insertions, deletions, and many other types of genetic change.
• Mutations can be harmful, neutral, or occasionally beneficial.
• Not every mutation causes a disorder.
• Mutations can affect how proteins are produced or function.
• Changes in protein function can affect cells, tissues, organs, and phenotype.
• Germline genetic changes can potentially be inherited.
• Somatic mutations are generally not passed to offspring.
• Genetic does not always mean inherited.
• New mutations can arise without a previous family history.
• Autosomal recessive disorders typically require two disease-associated recessive alleles.
• Unaffected carriers can pass recessive alleles to their children.
• Two carriers of a simple autosomal recessive disorder have a 25% predicted probability of an affected child per pregnancy.
• Cystic fibrosis is an example of an autosomal recessive disorder.
• Sickle cell disease is associated with variants in the HBB gene.
• Autosomal dominant disorders may occur when one disease-associated allele is inherited.
• Huntington disease is an example of autosomal dominant inheritance.
• Some disorders show X-linked inheritance.
• Hemophilia A and Duchenne muscular dystrophy are examples of X-linked disorders.
• Some genetic conditions involve changes in chromosome number rather than one gene.
• Down syndrome is usually associated with trisomy 21.
• Nondisjunction can produce cells with abnormal chromosome numbers.
• Pedigrees can help identify inheritance patterns.
• Dominant does not mean common.
• Recessive does not mean weak or mild.
• Genetic probabilities describe likelihood rather than guaranteed outcomes.
• Genetic testing can provide information about genes and chromosomes.
• Genetic counseling can help individuals and families understand inheritance and testing information.
• Genetic disorders can have medical, practical, emotional, and financial impacts.
• The effects of the same genetic disorder can vary between individuals.
• Genetic information raises important questions about privacy, testing, and informed decision-making.
• Understanding genetic disorders connects DNA, mutations, proteins, chromosomes, inheritance, probability, and human health.